Transistor, circuit board, display and electronic equipment

ABSTRACT

A transistor having at least one of a source electrode and a drain electrode being formed of a porous film is described. The transistor maintains its characteristics even after being subjected to a high temperature and high humidity environment. The transistor may be used in a circuit board, a display and electronic equipment.

RELATED APPLICATION INFORMATION

This application claims priority to Japanese Application No. 2004-32572,filed Feb. 9, 2004, whose contents are expressly incorporated herein byreference.

FIELD OF THE INVENTION

Aspects of the present invention relate to a transistor, a circuitboard, a display, and electronic equipment.

BACKGROUND

The development of transistors employing organic materials (forinstance, organic semiconductor materials) that exhibit semi-conductingelectrical conductivity has recently progressed. These transistors haveadvantages such as being thinner, lighter, having lower material costs,and so forth, supporting their use as a switching element of a flexibledisplay and so forth. Transistors have been proposed in which a sourceelectrode and a drain electrode are formed on a substrate while anorganic semiconductor layer, a gate insulating layer and a gateelectrode are deposited above these electrodes in that order. Thesetransistors can be fabricated in the atmosphere by using vapordeposition or a coating method.

However, the fact that these transistors can be fabricated in anatmosphere does not necessarily lead to the stable operation thereof inthe atmosphere. Specifically, in the atmosphere, the organicsemiconductor layer is doped with oxygen and water, which increases anoff-state current, or in contrast increases traps so as to cause thedeterioration of subthreshold swing (S value) and threshold voltage(Vth). A method of encapsulating a transistor has been proposed as amethod of preventing an organic semiconductor layer from being dopedwith oxygen and water in the atmosphere.

Specifically, a transistor is fabricated on a glass substrate and thenan encapsulating film is deposited in vacuum so as to cover thetransistor, thereby encapsulating it. This prevents the organicsemiconductor layer from being doped with oxygen and water, and thus thecharacteristics of the transistor can be stabilized. Also, a method hasbeen proposed in which a multilayer of inorganic films and polymer filmsalternately deposited is used as an encapsulating film for furtherenhancing the encapsulating characteristic.

However, the methods of encapsulating a transistor result in anincreased cost for the transistor because vacuum treatment and heattreatment are required. In addition, in the case of using a plasticsubstrate as a substrate, a gas barrier film also needs to be formed onthe plastic substrate side because of the low gas-barrier characteristicof plastic materials, thereby further increasing costs.

Also, if a non-flexible component material (for example, silicon oxideor silicon nitride) is used as an encapsulating film, the flexibility ofa transistor may be reduced. Even if these films are sufficiently thin,cracks tend to be generated in the films when a substrate is bent, ortemperature or humidity varies. It, therefore, is difficult to achievehigh reliability. Accordingly, a technique in which a gate electrode iscomposed of a metal film formed by vapor deposition has been proposed,for example, as a method of encapsulating a transistor at lower costs.Since a metal film formed by vapor deposition is a dense film andtherefore exhibits a comparatively high barrier characteristic againstwater and oxygen, forming the metal film as a gate electrode to cover achannel part enables the metal film to function as an encapsulatingmaterial.

However, although the transistor thus fabricated exerts a comparativelyhigh encapsulating characteristic, it is impossible to obtain a completeencapsulating. Therefore, if a transistor is subjected to a hightemperature and high humidity environment and oxygen and water enter theinside thereof once, the high encapsulating characteristic functionsadversely. That is, even when the transistor is brought back to a lowtemperature and low humidity environment, the oxygen and water are notdischarged outside the transistor but remain in the inside thereof for along time. Oxygen and water trapped therein deteriorate thecharacteristics of the transistor.

The following is a list of relevant documents related to theapplication: U.S. Pat. No. 6,150,191, U.S. Pat. No. 5,574,291,International Patent Publication No. 0147043, International PatentPublication No. 0147045, and Yong Qiu et al. (five coauthors), “H20effect on the stability of organic thin-film field-effect transistors”,APPLIED PHYSICS LETTERS, Unites States, American Institute of Physics,2003 Aug. 25, Vol. 83, No. 8, p. 1644-1646.

SUMMARY

Aspects of the present invention provide a transistor that can maintainits high characteristics even after being subjected to a hightemperature and high humidity environment. Aspects of the invention alsorelate to a circuit board, a display and electronic equipment that mayinclude the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams showing a thin film transistor of aspectsof the present invention, where FIG. 1A is a longitudinal sectionalview, and FIG. 1B is a plan view.

FIG. 2 is a plan view showing an embodiment of a circuit board ofaspects of the present invention.

FIG. 3A through 3E are diagrams (longitudinal sectional views) forexplaining a method of forming a thin film transistor part (line A-A inFIG. 2) of the circuit board of aspects of the present invention.

FIGS. 4F through 4I are diagrams (longitudinal sectional views) forexplaining the method of forming a thin film transistor part (line A-Ain FIG. 2) of the circuit board of aspects of the present invention.

FIG. 5 is a longitudinal sectional view showing an embodiment ofapplying the circuit board of aspects of the present invention to anelectrophoretic display.

FIG. 6 is a perspective view showing an embodiment of applying a displaydevice of aspects of the present invention to an electronic paper.

FIGS. 7A and 7B are diagrams showing an embodiment of applying thedisplay device of aspects of the present invention to a display.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description is divided into sections to assist the reader.

Overview

A transistor of at least one aspect of the present invention maycomprise: a source electrode and a drain electrode; an organicsemiconductor layer provided in contact with the source electrode andthe drain electrode; a gate insulating layer provided in contact withthe organic semiconductor layer; and a gate electrode insulated from thesource electrode and the drain electrode by intermediary of the gateinsulating layer, wherein at least one of the gate electrode, the sourceelectrode and the drain electrode is composed of a porous film composedmainly of an electrically conductive material. With this structure, thetransistor can maintain its high characteristics even after beingsubjected to an environment having high temperature and high humidity.

In the transistor of aspects of the present invention, the transistormay or may not have a top gate structure and the gate electrodeincluding a porous film. With this structure, the transistor canmaintain its high characteristics even after being subjected to a hightemperature and high humidity environment.

In the transistor of aspects of the present invention, porosity of theporous film may range from 20% to 85%. This porosity can more surelydischarge oxygen and water that have entered inside, and can adequatelyprevent a decrease in conductivity of the porous film.

In the transistor of aspects of the present invention, the porous filmmay or may not have a carbon atom or a carbon compound in a pore of theporous film. This enables the inner surfaces of the pores to be endowedwith hydrophobicity, and therefore can prevent dew condensation(accumulation) from being generated when water or the like passesthrough the pores. Thus the transformation and deterioration of theporous film can adequately be prevented.

In the transistor of aspects of the present invention, the porous filmmay or may not be formed by using a liquid material including anelectrically conductive particle. This may facilitate the formation of aporous film having an intended porosity.

In the transistor of aspects of the present invention, the conductiveparticle may or may not comprise as a main component at least one ofgold, silver, copper, platinum, palladium, nickel, and an alloyincluding these elements. These substances are preferable because ofhigh electrical conductivity.

In the transistor of aspects of the present invention, the liquidmaterial may or may not comprise the conductive particle and adispersion medium including water. In the transistor of the presentinvention, the porous film may or may not be formed by using a coatingmethod. Thus, the porous film can be formed more easily.

In the transistor of aspects of the present invention, the coatingmethod may or may not include an ink jet method. The ink jet methodpermits the formation of a porous film with high dimensional accuracy.In the transistor of aspects of the present invention, the porous filmmay be obtained through a heating process. This heating process caneasily and surely remove substances that may be included in the liquidmaterial and should be removed.

In the transistor of aspects of the present invention, heatingtemperature in the heating process may be equal to or smaller than 200degrees centigrade. This can prevent the lowering of the porosity.

In the transistor of aspects of the present invention, the conductivematerial may or may not include a metal material and/or a metal oxidematerial as a main component. Thus, the porous film having highconductivity may be formed.

In the transistor of aspects of the present invention, the organicsemiconductor layer may include a polymer including arylamine or aderivative of the polymer. The organic semiconductor layer composed ofsuch an organic semiconductor material may have high water-resistanceand oxidation-resistance characteristics. Therefore, in this example,the organic semiconductor layer is chemically stable particularly andthe quality deterioration thereof is avoided even when it is temporarilysubjected to a high temperature and high humidity environment.

In the transistor of aspects of the present invention, the organicsemiconductor layer may be composed mainly of a copolymer includingfluorene and bithiophene, or a derivative of the copolymer.

The organic semiconductor layer composed of such an organicsemiconductor material may exhibit high water-resistance andoxidation-resistance characteristics. Accordingly, the organicsemiconductor layer is chemically stable and deterioration of thequality of the layer is avoided even when a transistor is temporarilysubjected to a high temperature and high humidity environment.

In the transistor of aspects of the present invention, the organicsemiconductor layer may be formed by using an ink jet method. With theink jet method, the organic semiconductor layer having an intended shapecan be formed with high dimensional accuracy.

In the transistor of aspects of the present invention, both of thesource electrode and the drain electrode may be formed in a comb teethshape and in such a manner that the teeth interdigitate with each other.This form can prevent the increase of the area of an overlapping partbetween the gate electrode and the source and drain electrodes, and thusthe characteristics of the transistor can further be enhanced.

A circuit board of aspects of the present invention comprises aplurality of the transistors of the present invention. This can providea highly reliable circuit board. In the circuit board of the presentinvention, at least a part of the gate electrode provided for thetransistors is composed of a common electrode. This structurefacilitates the formation of a gate electrode, reducing themanufacturing time of the circuit board. In the circuit board of thepresent invention, the common electrode is preferably almost linear.

A display of the present invention comprises: the circuit board of thepresent invention; pixel electrodes connected to each of thetransistors; a counter electrode provided so as to face the pixelelectrodes; and a display medium interposed between the counterelectrode and the pixel electrodes. This structure allows the provisionof a highly reliable display.

The display of aspects of the present invention may further comprise aprotective film provided closer to the circuit board than the displaymedium so as to cover the transistors and the pixel electrodes. Forexample, this structure can adequately prevent shear stress from beingplaced on the transistor, the display medium, and so forth. In thedisplay of the present invention, the display medium may or may notcomprise an organophilic liquid that dissolves or swells a materialforming the organic semiconductor layer.

In the display of aspects of the present invention, the protective filmmay or may not have a function to prevent the organophilic liquid fromdiffusing toward the circuit board. This can avoid the dissolution andswelling of the organic semiconductor layer, and therefore canadequately prevent the deterioration of characteristics of thetransistor. In the display of aspects of the present invention, theprotective film may be formed mainly of a hydrophilic polymer material.According to this, even if the organophilic liquid flows out from thedisplay medium, the organophilic liquid can be prevented from passingthrough the protective film so as to enter the circuit board.

In the display of the present invention, the polymer material mayinclude as a main component a polymer including vinyl alcohol. Thesesubstances have particularly high hydrophilicity. Therefore, even if theorganophilic liquid flows out from the display medium, the organophilicliquid can be prevented from passing through the protective film so asto enter the circuit board particularly effectively. Electronicequipment of aspects of the present invention may include a display inaccordance with aspects of the present invention. Thus, the electronicequipment with high reliability can be obtained.

Thin Film Transistor

First, the structure of a thin film transistor of the present inventionis described.

FIGS. 1A and 1B are diagrams showing a thin film transistor having afirst structure. FIG. 1A is a longitudinal sectional view, and FIG. 1Bis a plan view. In the following description, the upper side in FIG. 1Acorresponds to the upper side of the thin film transistor, while thelower side corresponds to the lower side thereof. A thin film transistor1 shown in FIGS. 1A and 1B is provided on a substrate 2, and comprises asource electrode 3 (source electrode fingers 3 a) and a drain electrode4 (drain electrode fingers 4 a), an organic semiconductor layer (organiclayer) 5, a gate insulating layer 6, and a gate electrode 7 that aredeposited in that order from the substrate 2 side.

Specifically, in the thin film transistor 1, both the source electrode 3and drain electrode 4 are formed on the substrate 2 in comb teeth and insuch a manner that the teeth interdigitate with each other. Theinterdigitation may occur on a one-to-one basis or other interval(two-to-two, one-to-two, etc.). The organic semiconductor layer 5 isprovided so as to cover and be in contact with the electrodes 3 and 4.The gate insulating layer 6 is provided on and in contact with theorganic semiconductor layer 5. On the gate insulating layer 6, the gateelectrode 7 is provided so as to overlap, with the intermediary of thegate insulating layer 6, a region in which the teeth of the source anddrain electrodes 3 and 4 interdigitate with each other.

More specifically, the source electrode 3 and the drain electrode 4 havea plurality of electrode fingers 3 a and 4 a arranged at certainintervals, respectively, so as to have a comb teeth shape as a whole.Furthermore, the source and drain electrodes 3 and 4 are provided insuch a manner that the electrode fingers 3 a and 4 a are alternatelyarranged (in varying degrees as described above).

Such a thin film transistor 1 has a structure in which the source anddrain electrodes 3 and 4 are provided closer to the substrate 2 than thegate electrode 7, with the gate insulating layer 6 being between theelectrodes 3 and 4, and the gate electrode 7. That is, the thin filmtransistor 1 has a top gate structure.

Each element making up the thin film transistor 1 will be describedbelow sequentially.

The substrate 2 supports each layer (each element) of the thin-filmtransistor 1. The following substrates can be used as the substrate 2,for example: a glass substrate; a plastic substrate (resin substrate)composed of polyimide, polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), polymethylmethacrylate (PMMA), polycarbonate (PC),polyethersulphone (PES), aromatic polyester (liquid crystal polymer), orthe like; a quartz substrate; a silicon substrate; and a galliumarsenide substrate. In the case of endowing the thin film transistor 1with flexibility, a plastic substrate is selected as the substrate 2.

A underlying layer may be provided on the substrate 2. The underlyinglayer is provided, for example, with a view to preventing the diffusionof ions from the surface of the substrate 2, or to improving theadhesiveness (bonding characteristic) between the source and drainelectrodes 3 and 4 and the substrate 2. As a component material of aunderlying layer, although there is no limitation, silicon dioxide(SiO2), silicon nitride (SiN), polyimide, polyamide, a polymer that iscross-linked so as to be insolubilized, or the like may be used.

The electrode fingers 3 a and 4 a of the source and drain electrodes 3and 4 are provided on the substrate 2 along the channel length Ldirection in such a manner to be arranged alternately at certainintervals.

In the thin film transistor 1, out of the organic semiconductor layer 5,regions between the electrode fingers 3 a of the source electrode 3 andthe electrode fingers 4 a of the drain electrode 4 are channel regions51 in which carriers move. Also, the length of movement direction ofcarriers in regions between the electrode fingers 3 a of the sourceelectrode 3 and the electrode fingers 4 a of the drain electrode 4, thatis, the distance between each of the electrode fingers 3 a and 4 acorresponds to the channel length L. The product of the length ω of thedirection perpendicular to the channel length L direction and the numberof intervals (gaps) N between the electrode fingers 3 a and 4 a is thechannel width W.

In the thin film transistor 1 in which the source and drain electrodes 3and 4 are formed in comb teeth and regions between the electrode fingers3 a and 4 a are channel regions, the width A of the electrode fingers 3a and 4 a determines the area of overlapping portions between the sourceand drain electrodes 3 and 4, and the gate electrode 7.

Furthermore, in the present embodiment, the source and drain electrodes3 and 4 may be formed using a resist layer formed by photolithography asa mask, as described later. The width A of the electrode fingers 3 a and4 a depends on the accuracy of photolithography. However, the narrowingof the electrode fingers is possible since the accuracy ofphotolithography is high.

Thus, even if the gate electrode 7 is formed by an ink jet method asdescribed later and therefore has a comparatively large width, the areaof overlapping portions between the gate electrode 7 and the source anddrain electrodes 3 and 4 can be prevented from increasing. According tothis, the capacitance of the gate can be reduced in the thin filmtransistor 1. As a result, good characteristics (for instance, aswitching characteristic) may be exhibited.

As above, since there is no need to form the gate electrode 7 in aminute shape in the present embodiment, the range of options in aforming method thereof is widened. Thus, the thin film transistor 1having a good characteristic is obtained even if various coating methods(including but not limited to the ink jet method) are used for formingthe gate electrode 7.

A component material of these source and drain electrodes 3 and 4 is notparticularly limited as long as it has electrical conductively. Forexample, one or a combination of two or more of the following substancescan be used as the material: conductive materials such as Pd, Pt, Au, W,Ta, Mo, Al, Cr, Ti, Cu, and an alloy including these elements;conductive oxides such as ITO, FTO, ATO and SnO2; carbon materials suchas a carbon black, a carbon nanotube and a fullerene; and conductivepolymer materials such as a polyacetylene, a polypyrrole, apolythiophene like a poly-ethylenedioxythiophene (PEDOT), a polyaniline,a poly(p-phenylene), a polyfluorene, a polycarbazole, a polysilane, anda derivative of these substances. The conductive polymer material istypically used after being doped with a high molecular weight materialof an iron oxide, iodine, an inorganic acid, an organic acid, or apolystyrene sulfonic acid so as to be endowed with electricalconductivity. Of the above substances, a material composed mainly of Ni,Cu, Co, Au, Pd, or an alloy including these elements is may, in someexamples of the invention, be used as a component material of the sourceand drain electrodes 3 and 4. In the case of forming the source anddrain electrodes 3 and 4 by using these metal materials, the electrodes3 and 4 of high deposition accuracy can be formed at low costs by usingelectroless plating in a forming process of the electrodes 3 and 4 to bedescribed later, enabling the fabrication of the thin film transistor 1having high characteristics (for example, switching characteristics).

The thickness (in average) of the source and drain electrodes 3 and 4may be, but not limited to, about 30-300 nm, and is more preferablyabout 50-150 nm.

The width A of the electrode fingers 3 a and 4 a is preferably 20 μm orless, and is more preferably 10 μm or less, although greater widths Amay be used.

Also, the distance (separation distance) between the source electrodefinger 3 a and the drain electrode finger 4 a, that is, the channellength L is preferably about 2-20 μm, and is more preferably about 3-10μm, although other channel lengths may be used. The smaller channellength L permits the control of a larger drain current, and enables thecapacitance of the gate electrode to be reduced. However, if the channellength L is smaller than the above lower limit, a highly accuratephotolithography techniques may be required for patterning theelectrodes, causing a rise in associated manufacturing costs. Inaddition, even if the small channel length L is achieved, expectedeffects are not always obtained because of the contact resistancebetween the source electrode and the organic semiconductor layer in manycases. In contrast, if the channel length L is larger than the aboveupper limit, a drain current may be made small and, therefore, thecharacteristics of the thin film transistor 1 may be insufficient.

The channel width W is preferably about 0.1-5 mm, and is more preferablyabout 0.3-3 mm. If the channel width W is smaller than the above lowerlimit, a drain current becomes small and therefore the characteristicsof the thin film transistor 1 may be insufficient. In contrast, thechannel width W is larger than the above upper limit, the size of thethin film transistor 1 increases while the increase of the parasiticcapacitance and the increase of a leak current to the gate electrode 7through the gate insulating layer 6 may be caused.

The organic semiconductor layer 5 is provided on and in contact with thesubstrate 2 so as to cover the source and drain electrodes 3 and 4.

The organic semiconductor layer 5 is composed mainly of an organicsemiconductor material (organic material exhibiting semiconductingelectrical conductivity).

The organic semiconductor layer 5 is preferably oriented so as to bealmost parallel with the channel length L direction at least in channelregions 51. This enhances the carrier mobility in the channel regions51, with the result that the operation speed of the thin film transistor1 is further enhanced.

For example, examples of an organic semiconductor material include lowmolecular weight organic semiconductor materials such as a naphthalene,anthracene, tetracene, pentacene, hexacene, phtalocyanine, perylene,hydrazone, triphenylmethane, diphenylmethane, stilbene, aryl vinyl,pyrazoline, triphenylamine, triarylamine, oligothiophene, and derivativeof these substances. Also, examples of an organic semiconductor materialinclude polymer organic semiconductor materials (conjugated polymermaterial) such as a poly(N-vinylcarbazole), polyvinylpyrene,polyvinylanthracene, polythiophene, polyhexylthiophene,poly(p-phenylenevinylene), polythienylenevinylene, polyarylamine, pyreneformaldehyde resin, ethylcarbazole formaldehyde resin,fluorene-bithiophene copolymer, fluorene-arylamine copolymer, andderivative of these substances. One or a combination of two or more ofthe above substances can be used. Particularly, it is preferable to usea material composed mainly of a polymer organic semiconductor material(conjugated polymer material). In a conjugated polymer material, themobility of carriers is particularly high because of the particulardistribution of electron clouds therein. Such a polymer organicsemiconductor material can be deposited with a simple method and can beoriented comparatively easily.

Of these materials, an organic semiconductor material is may be composedmainly of at least one of a copolymer including a fluorene andbithiophene such as a fluorene-bithiophene copolymer, a polymerincluding an arylamine such as a polyarylamine or fluorine-arylaminecopolymer, and a derivative of these substances. An organicsemiconductor material is more preferably composed mainly of at leastone of a polyarylamine, a fluorene-bithiophene copolymer, and aderivative of these substances. The organic semiconductor layer 5composed of such an organic semiconductor material has highwater-resistance and oxidation-resistance characteristics. Therefore,the organic semiconductor layer 5 is chemically stable and the qualitydeterioration thereof is avoided, even when it is temporarily subjectedto a high temperature and high humidity environment.

The organic semiconductor layer 5 including a polymer organicsemiconductor material as a main material can be formed in a thinned andlightened manner, and is superior in flexibility. Therefore, it issuitably applied to a thin film transistor used as a switching elementof a flexible display and so forth.

The thickness (in average) of the organic semiconductor layer 5 ispreferably about 0.1-1000 nm, and is more preferably about 1-500 nm.Also, the thickness is further preferably about 10-100 nm.

The organic semiconductor layer 5 need not necessarily cover the sourceand drain electrodes 3 and 4. It is sufficient that the organicsemiconductor layer 5 is provided at least on the regions (channelregions 51) between the source and drain electrodes 3 and 4.

The gate insulating layer 6 is provided on and in contact with theorganic semiconductor layer 5. The gate insulating layer 6 insulates thegate electrode 7 from the source and drain electrodes 3 and 4.

The gate insulating layer 6 is preferably composed mainly of an organicmaterial (particularly organic polymer material). The gate insulatinglayer 6, composed mainly of an organic polymer material, is easy to formand can achieve higher adhesiveness with the organic semiconductor layer5.

For example, examples of such an organic polymer material include:acrylic resin such as polystyrene, polyimide, polyamideimide,polyvinylphenylene, polycarbonate (PC) and polymethylmethacrylate(PMMA); fluorine resin such as polytetrafluoroethylene (PTFE); phenolresin such as polyvinylphenol and novolac resin; and olefin resin suchas polyethylene, polypropylene, polyisobutylene and polybutene. One or acombination of two or more of these materials can be used.

The thickness (in average) of the gate insulating layer 6 is preferably,but not limited to, about 10-5000 nm, and is more preferably 100-1000nm. If the thickness of the gate insulating layer 6 is in the aboverange, the source and drain electrodes 3 and 4 are surely insulated fromthe gate electrode 7, while the size of the thin film transistor 1 (thethickness thereof particularly) can be prevented from increasing. Thegate insulating layer 6 is not limited to a single layer but may have amulti-layered structure.

Also, an inorganic insulating material such as SiO2 may be used as acomponent material of the gate insulating layer 6, for example. In thiscase, a solution of polysilicate, polysiloxane or polysilazane isapplied and then the coating film is heated in the presence of oxygen orwater vapor, whereby SiO2 can be obtained as the gate insulating layer 6from the solution material. Alternately, a metal alkoxide solution isapplied and then is heated in an oxygen atmosphere, whereby an inorganicinsulating material can be obtained (this method is known as a sol-gelmethod).

Gate electrode 7 may be provided aAbove the gate insulating layer 6where it is insulated from the source and drain electrodes 3 and 4 bythe intermediary of the gate insulating layer 6.

In at least one embodiment, the gate electrode 7 may be composed of aporous film composed mainly of a conductive material.

If the gate electrode is composed of a metal film formed by vapordeposition, although such a gate electrode exhibits a comparatively highencapsulating characteristic as described above, it is impossible toachieve a complete encapsulating characteristic. In such a thin filmtransistor, therefore, if oxygen and water once enters the insidethereof, the oxygen and water are not discharged outside the thin filmtransistor but remain therein for a long time. Such a thin filmtransistor then exhibits a problem in that oxygen and water, accumulatedtherein, deteriorates the characteristics of the transistor.

In contrast, in at least some aspects of the present invention, at leastone of a source electrode, drain electrode and gate electrode (the gateelectrode 7 in the present embodiment) may be composed of a porous filmcomposed mainly of a conductive material, whereby the gate electrode 7has high air permeability. As a result, even if the thin film transistor1 is subjected to a high temperature and high humidity environmenttemporarily, the oxygen and water that have entered the inside thereofare rapidly discharged outside the thin film transistor 1 when the thinfilm transistor 1 is brought back to a low temperature and low humidityenvironment. Thus, since oxygen and water are prevented from beingaccumulated in the inside of the thin film transistor 1 of the presentinvention, the characteristics of the thin film transistor 1 areadequately maintained.

The porosity of the porous film is preferably, but not limited to, about20-85%, and is more preferably about 40-85%. Also, it is furtherpreferably about 60-75%. According to this, oxygen and water that haveentered the thin film transistor 1 are more readily dischargedtherefrom. If the porosity is beyond the above upper limit, theelectrical conductivity of the porous film may be extremely lowereddepending on the kind or the like of the conductive material.

For example, examples of a conductive material (metal material or metaloxide material) constituting a porous film include: Ag, Pd, Pt, Au, W,Ta, Mo, Al, Cr, Ti, Cu, Ni, an alloy including these elements, indiumtin oxide (ITO), indium oxide (IO), indium zinc oxide (IZO), antimonytin oxide (ATO), and tin dioxide (SnO2). One or a combination of two ormore of these substances can be used. In addition to these substances,for example, such a conductive polymer material as referenced above as amaterial for the source and drain electrodes 3 and 4 may be used as aconductive material.

Of these substances, at least one of Au, Ag, Cu, Pt, Pd, Ni, and analloy including these elements is preferably used as a main component.These substances are preferable because of high electrical conductivity.The thickness (in average) of the porous film (gate electrode 7) ispreferably, but not limited to, about 0.1-5000 nm, and is morepreferably about 1-5000 nm. Also, the thickness may further preferablyrange between 10-5000 nm.

Although other substances may not exist in the pores of the porous filmsubstantially, it is preferable that carbon atoms or carbon compoundsexist therein. This enables the inner surfaces of the pores to beendowed with hydrophobicity, and therefore the generation of dewcondensation (accumulation) when water or the like passes through thepores can be avoided. As a result, the transformation and deteriorationof a porous film can be adequately prevented.

For example, graphite, hydrogen carbon, etc. carbon compounds. Thesecarbon compounds may arise (be generated) in a forming process of afirst line 91 (gate electrode 7) to be described later, or may beprovided in pores after a porous film is formed, for example.

In the thin film transistor 1 as above, the amount of a current flowingbetween the source and drain electrodes 3 and 4 is controlled bychanging a voltage applied to the gate electrode 7.

Specifically, in an OFF state in which a voltage is not applied to thegate electrode 7, even if a voltage is applied between the sourceelectrode 3 (source electrode fingers 3 a) and the drain electrode 4(drain electrode fingers 4 a), no more than a minute current flows sincefew carriers exist in the organic semiconductor layer 5. Meanwhile, inan ON state in which a voltage is applied to the gate electrode 7,movable charges (carriers) are induced in a portion of the organicsemiconductor layer 5 that faces the gate insulating layer 6, and thus apath of carriers is formed in the channel region 51. When a voltage isapplied between the source and drain electrodes 3 and 4 in this state, acurrent flows through the channel region 51.

In the various embodiments, a structure has been described in which bothof the source and drain electrodes 3 and 4 are formed in comb teeth andthe teeth interdigitate with each other. However, the shape of theseelectrodes 3 and 4 are not limited thereto. For example, both of theelectrodes 3 and 4 may be formed in a substantially rectangle and may bejuxtaposed to each other at a certain interval.

Also, in the various embodiments, thin film transistor 1 having the gateelectrode 7 may be composed of a porous film. However, at least one of asource electrode, drain electrode and gate electrode may be composed ofa porous film.

Furthermore, the thin film transistor of aspects of the presentinvention may also be applied to, besides a top gate structure, a bottomgate structure thin film transistor.

(Circuit Board)

One example of the structure of a circuit board including a plurality ofthe above-described thin film transistors 1 will now be described. FIG.2 is a plan view showing an embodiment of a flexible circuit board ofaspects of the present invention. In the following description, the nearside of FIG. 2 corresponds to the upper side of the circuit board, whilethe back side corresponds to the lower side thereof. A circuit board 10shown in FIG. 2 comprises the substrate 2, pixel electrodes 41, the thinfilm transistors 1, connection terminals 8 and lines 9. Each of theelements 41, 1, 8 and 9 is provided on the substrate 2.

The substrate 2 is a support body for supporting each of the elements41, 1, 8 and 9 provided thereon. The pixel electrodes 41 constituteelectrodes of one side that apply a voltage for driving each pixel whenan electrophoretic display 20 to be described later is built by usingthe circuit board 10. The pixel electrodes 41 are arranged in a matrix.

Each pixel electrode 41 is connected to the drain electrode 4 includedin each of the thin film transistors 1 that are arranged in a matrix.Therefore, controlling the operation of the thin film transistors 1enables the driving of each pixel to be controlled in theelectrophoretic display 20 to be described later. The connectionterminals 8 comprise a plurality of first terminals 81 and a pluralityof second terminals 82.

Each of the first terminals 81 and each of the second terminals 82constitute a terminal to be connected to a driving IC.

Also, the lines 9 are constituted by a plurality of first lines 91 and aplurality of second lines 92 provided relatively perpendicularly to thelines 91.

In the present embodiment, the gate electrode 7 for the thin filmtransistors 1 arranged in one row is a common electrode althoughseparate electrodes may be used as well. This common electrodeconstitutes the first line 91. Thus the first lines 91 are substantiallystraight. One end of each of the first lines 91 is connected to one ofthe first terminals 81.

Such a configuration of the first lines 91 enables the gate electrodes 7of a certain number (as many as needed) to be formed at the same time.As a result, the manufacturing time for the circuit board 10 can beshortened.

Also, one end of each of the second lines 92 may be connected to one ofthe second terminals 82. The source electrodes 3 of a plurality of thethin film transistors 1 are connected to halfway points of the secondlines 92.

Any substance can be used as a component material of the pixelelectrodes 41, the connection terminals 8 (first terminals 81 and secondterminals 82) and the second lines 92 as long as the substance haselectrical conductivity. For example, the same substances as referencedabove as a component material of the source and drain electrodes 3 and 4can be used. Accordingly, in a manufacturing process of a circuit boardto be described later, the source and drain electrodes 3 and 4, thepixel electrodes 41, the connection terminals 8 and the second lines 92can be formed at the same time in the same process.

As a component material of the first lines 91, in addition to thesubstances referenced above as a component material of theabove-described gate electrode 7, the same substances as referencedabove as a component material of the source and drain electrodes 3 and 4can be used.

Circuit board 10 including a plurality of the thin film transistors 1may be manufactured through the following processes, for example.

Method Of Manufacturing Circuit Board

FIGS. 3A through 3E and 4F through 4I are diagrams (longitudinalsectional views) for explaining a method of forming thin film transistor1 part (line A-A in FIG. 2) of the circuit board 10. In the followingdescription, the upper side in FIGS. 3A through 3E and 4F through 4Icorresponds to the upper side of the thin film transistor, while thelower side corresponds to the lower side thereof.

A first manufacturing method of the circuit board 10 comprises: (Al) astep of forming source electrodes, drain electrodes, pixel electrodes,lines and connection terminals; (A2) a step of removing organicsubstances; (A3) a step of forming an organic semiconductor layer; (A4)a step of forming a gate insulating layer; and (A5) a step of forminglines (gate electrodes). Each step will be described below sequentially.

(A1) Step of Forming Source Electrodes, Drain Electrodes, PixelElectrodes, Lines and Connection Terminals

The source and drain electrodes 3 and 4, the pixel electrodes 41, theconnection terminals 8 and the second lines 92 can be formed by forminga film (conductive film) composed of a conductive material, andthereafter removing unwanted portions. The film composed of a conductivematerial is formed by, for example, chemical vapor deposition (CVD) suchas plasma CVD, thermal CVD or laser CVD, dry plating such as vapordeposition, sputtering or ion plating, wet plating such as electrolyticplating, immersion plating or electroless plating, spraying, a sol-gelmethod or metal organic decomposition (MOD) or other known processes. Inparticular, the conductive film is preferably formed by electrolessplating. The use of electroless plating enables the source and drainelectrodes 3 and 4, the pixel electrodes 41, the connection terminals 8and the second lines 92 to be easily formed with high depositionaccuracy at low costs without a large-scale apparatus such as a vacuumapparatus.

The case of using electroless plating for forming the source and drainelectrodes 3 and 4, the pixel electrodes 41, the connection terminals 8and the second lines 92 will be described below by way of example. It isappreciated that the various items found in step A1 may be formedsimultaneously or separately or in various combinations withoutdeparting from the scope of aspects of the invention.

(A1-1) First the substrate 2 such as that shown in FIG. 3A is prepared,and then the substrate 2 is cleaned with one or an adequate combinationof water (deionized water or the like), an organic solvent, etc. Thiscleaning improves the wettability of the substrate 2 to water and thusis easier for various treatment liquids described below to contact thesubstrate 2.

In the case of using a substrate composed of resin such as polyimide asthe substrate 2, it is preferable that for a surface of the substrate 2on which the source and drain electrodes 3 and 4, the pixel electrodes41, the connection terminals 8 and the second lines 92 are formed,adhesiveness improving treatment for enhancing the adhesiveness betweenthese elements and the substrate 2 is carried out prior to the presentstep (A1-1) (step (A1)).

This adhesiveness improving treatment (surface roughening) is carriedout by etching the surface of the substrate 2 with an etchant, and thentreating the surface with a treatment liquid including a reducing agent.

For example, a liquid containing a transition metal oxide such as CrO3or MnO2 and an inorganic acid such as a sulfuric acid or hydrochloricacid among others can be used as the etchant.

Meanwhile, it is preferable that an agent that does not contain alkalimetal elements substantially is used as the reducing agent used astreatment liquid, although alternative agents may be used. This avoidsthat alkali metal ions are trapped in the surface of the substrate 2,and thus preventing the diffusion (mixing) of alkali metal ions into theorganic semiconductor layer 5 to be formed in the later process. As aresult, the deterioration of the characteristics of the organicsemiconductor layer 5 can be prevented.

Ammonium compounds such as ammonium sulfite hydrate and ammoniumhypophosphite, hydrazine, etc. may be used as a reducing agent. Of thesesubstances, an ammonium compound is preferable as a main component of areducing agent, and ammonium sulfite hydrate is more preferable. Anammonium compound (particularly ammonium sulfite hydrate) is preferablebecause of excellent reducing properties.

(A1-2) Next, pre-treatment for forming a plated film 11 is carried outfor the substrate 2.

This pre-treatment is carried out by bringing a solution (surfactantsolution) including a cationic surfactant or an anionic surfactant intocontact with the substrate 2. This allows the cationic surfactant oranionic surfactant to be adhered to the surface of the substrate 2.

The surface of the substrate 2 is positively charged when a cationicsurfactant adheres thereto, while being negatively charged when ananionic surfactant adheres thereto. If the charge polarity of a catalystused in electroless plating is opposite to the charging of the substratesurface, the catalyst is easily adsorbed. As a result, the adhesivenessbetween the plated film 11 (source and drain electrodes 3 and 4, pixelelectrodes 41, connection terminals 8 and second lines 92) to be formedand the substrate 2 is enhanced.

For example, a method of immersing the substrate 2 in a surfactantsolution (immersion method) and a method of showering (spraying) thesubstrate 2 with a surfactant solution are listed as a method ofcontacting a surfactant solution with the substrate 2. The immersionmethod is preferable in particular. The immersion method enables a largeamount of the substrates 2 to be easily treated.

Although there are various methods for contacting a liquid with thesubstrate 2 as above, the case of using an immersion method as a methodof contacting a liquid will be described as representation in thefollowing processes.

Examples of a cationic surfactant include, for example, alkyl ammoniumchloride, benzalkonium chloride, benzethonium chloride and an stearicacid. One or a combination of two or more of these substances can beused.

The temperature of the surfactant solution for treatment is preferablyabout 0-70° C., and is more preferably about 10-40° C.

Also, the treatment time for the substrate 2 in a surfactant solution ispreferably about 10-90 seconds, and is more preferably about 30-60seconds.

The substrate 2 thus pre-treated is cleaned with using, for example,deionized water (ultrapure water), ion exchange water, distilled wateror reverse osmosis (RO) water.

(A1-3) Next, a catalyst is adsorbed (attached) to the surface of thesubstrate 2.

As a catalyst, Au, Ag, Pd, Pt, Ni, etc. are listed. One or a combinationof two or more of these elements can be used.

In the case of using Pd as a catalyst out of these elements, thesubstrate 2 is immersed in a colloidal suspension of a Pd alloy such asSn—Pd or a solution of an ionic Pd catalyst such as palladium chloride,thereby allowing a Pd alloy or ionic Pd catalyst to be adsorbed(attached) to the surface of the substrate 2. Thereafter elementsirrelevant to catalyzing are removed, whereby Pd is exposed on thesurface of the substrate 2.

For example, in the case of using an Sn—Pd colloidal suspension, thesubstrate 2 is immersed in the colloidal suspension and then is immersedin an acid solution. By doing so, Sn coordinated to Pd is dissolved soas to be removed, with the result that Pd is exposed on the surface ofthe substrate 2.

For example, a solution including an acid such as HBF4 and a reducingagent such as glucose, or a solution prepared by further adding asulfuric acid to the solution can be used as the acid solution.

The temperature of the solution including a catalyst for treatment ispreferably about 0-70° C., and is more preferably about 10-40° C.

Also, the treatment time for the substrate 2 in the solution including acatalyst is preferably from about 10 seconds to about 5 minutes, and ismore preferably from about 20 seconds to about 3 minutes.

Meanwhile, the temperature of the acid solution for treatment ispreferably about 0-70° C., and is more preferably about 10-40° C.

The treatment time for the substrate 2 in the acid solution ispreferably from about 10 seconds to about 5 minutes, and is morepreferably from about 30 seconds to about 3 minutes.

The substrate 2 to which a catalyst has been thus attached (adsorbed) iscleaned with using, for example, deionized water (ultrapure water), ionexchange water, distilled water or RO water.

(A1-4) Subsequently, as shown in FIG. 3B, the substrate 2 is immersed ina plating solution 13, thereby allowing metallic elements (elementalmetal) to precipitate on the surface of the substrate 2 to form theplated film 11.

As the plating solution 13 used for electroless plating, a liquid thatincludes a metal salt of a metal for forming the plated film 11 (thesource and drain electrodes 3 and 4, the pixel electrodes 41, theconnection terminals 8 and the second lines 92) and a reducing agent,but not substantially including any alkali metal ions, is preferablyused.

That is, when preparing the plating solution 13 by resolving at least ametal salt and a reducing agent into the solvent, substances that do notinclude an alkali metal as a component element thereof are preferablyused as a composition of the plating solution 13.

This prevents the mixing of alkali metal ions into the plated film 11 tobe formed. As a result, the diffusion (mixing) of alkali metal ions intothe organic semiconductor layer 5 to be formed in the later process isprevented, and thus the deterioration of the characteristics of theorganic semiconductor layer 5 can be prevented.

As a metal salt, for example, sulfate salt, nitrate salt, or the like ispreferably used.

As a reducing agent, for example, hydrazine and ammonium hypophosphitecan be used. Of these substances, at least one of hydrazine and ammoniumhypophosphite is preferably used as a main component of a reducingagent. By using these materials as a reducing agent under appropriatetemperature and pH of the plating solution, the deposition rate of theplated film 11 is made proper. Thus the thickness of the film can easilybe controlled so as to be within the optimum range of film thicknessrequired for the source and drain electrodes 3 and 4, the pixelelectrodes 41, the connection terminals 8 and the second lines 92.Furthermore, the resulting plated film 11 can also have an eventhickness and preferable surface property (good film surfacemorphology).

The amount of a metal salt included in the plating solution 13 (theamount of a metal salt added to a solvent) is preferably about 1-50 g/L,and is more preferably about 5-25 g/L. If the content of a metal salt istoo small, it may take a long time to form the plated film 11. Incontrast, even if the content of a metal salt is increased beyond theabove upper limit, an increase in effectiveness of the solution is notexpected.

The amount of a reducing agent included in the plating solution 13 (theamount of a reducing agent added to a solvent) is preferably about10-200 g/L, and is more preferably about 50-150 g/L. If the amount of areducing agent is too small, it may be difficult to efficiently reducemetal ions depending on the kind or the like of the reducing agent. Incontrast, even if the content of a reducing agent is increased beyondthe above upper limit, more effectiveness is not expected.

It is preferable that a pH adjuster (pH buffer) is further mixed (added)to such a plating solution 13. Accordingly, the pH of the platingsolution 13 is prevented or suppressed from lowering as the electrolessplating proceeds. As a result, the decrease of the deposition rate orchanges in the composition or characteristics of the plated film 11 caneffectively be prevented.

As such a pH adjuster, although various kinds can be used, an adjustercomposed mainly of at least one of ammonia water, trimethyl ammoniumhydride, and ammonium sulfide is preferable. Since these substancesexcel in buffering effect, the use of these materials as a pH adjustorprovides the above effectiveness more remarkably.

By immersing the substrate 2 having a catalyst attached thereto in theplating solution 13 described above, the electroless plating reaction ispromoted by means of the catalyst as nuclei, thus forming the platedfilm 11.

The pH of the plating solution 13 for the treatment is preferably about5-12, and is more preferably about 6-10.

The temperature of the plating solution 13 for the treatment ispreferably about 30-90° C., and is more preferably about 40-80° C.

The treatment time for the substrate 2 in the plating solution 13 ispreferably from about 10 seconds to about 5 minutes, and is morepreferably from about 20 seconds to about 3 minutes.

If the pH and temperature of the plating solution 13 and the treatmenttime with the plating solution 13 are within the above ranges, thedeposition rate becomes particularly proper, and accordingly the platedfilm 11 having an even thickness can be formed with high precision.

Note that the thickness of the plated film 11 to be formed can becontrolled by arranging the plating conditions such as operationtemperature (temperature of a plating solution), operation time (platingtime), the amount of a plating solution, the pH of a plating solution,or the number of plating processes (the number of turns).

Furthermore, an additive such as a complexing agent or stabilizing agentmay be added to the plating solution 13 adequately, if necessary.

As a complexing agent, for example, carboxylic acids such as anethylenediamine tetra acetic acid and acetic acid, oxycarboxylic acidssuch as a tartaric acid and citric acid, an aminocarboxylic acid such asglycine, an amine such as triethanolamine, and polyhidric alcohols suchas glycerin and sorbitol can be used.

As a stabilizing agent, for example, 2, 2′-bipyridyl, cyanide,ferrocyanide, phenanthroline, thiourea, mercaptbenzothiazole, andthioglycolic acid can be used.

The substrate 2 on which the plated film 11 has been thus formed iscleaned by using, for example, deionized water (ultrapure water),ion-exchange water, distilled water or RO water.

(A1-5) Next, on the plated film 11 formed is a resist layer 12 having ashape corresponding to the shapes of the source and drain electrodes 3and 4, the pixel electrodes 41, the connection terminals 8 and thesecond lines 92.

First, as shown in FIG. 3C, a resist material 12′ is applied (provided)on the plated film 11. Then, the resist material 12′ maybe exposed via aphoto mask corresponding to the shapes of the source and drainelectrodes 3 and 4, the pixel electrodes 41, the connection terminals 8and the second lines 92, and then being developed with a developer.These processes form the resist layer 12 patterned into a shapecorresponding to the shapes of the source and drain electrodes 3 and 4and the second lines 92, and also the pixel electrodes 41 and theconnection terminals 8 that are not shown in the drawing, as shown inFIG. 3D.

(A1-6) Next, using the resist layer 12 as a mask, unnecessary portionsof the plated film 11 are removed by etching as shown in FIG. 3E.

This etching can be carried out by using one or a combination of two ormore of plasma etching, reactive etching, beam etching, photo-assistetching, other physical etching methods, wet etching, other chemicaletching methods, etc. The wet etching is preferably used, of theseetching methods. Thus, the etching process can be carried out withsimple apparatuses and steps without using any large-scale apparatussuch as a vacuum apparatus.

As an etchant used for wet etching, for example, a solution includingferric chloride, and a solution including a sulufric acid, nitric acid,or acetic acid can be used.

(A1-7) Subsequently, the resist layer 12 is removed. Thereby the sourceand drain electrodes 3 and 4 and the second lines 92 shown in FIG. 4F,and also the pixel electrodes 41 and the connection terminals 8 that arenot shown in the drawing, are obtained.

A resist remover is preferably used for removing the resist layer 12,and besides, for example, the above physical etching methods can also beused.

As described above, by using photolithography and etching incombination, the source and drain electrodes 3 and 4, the pixelelectrodes 41, the connection terminals 8 and the second lines 92 withhigh dimensional accuracy can easily and surely be formed.

Therefore, the width A of the source electrode fingers 3 a and the drainelectrode fingers 4 a, and the distance (channel length L) between thesource electrode finger 3 a and the drain electrode finger 4 a can beset comparatively shorter, which permits the fabrication of the thinfilm transistor 1 with a low absolute value of the threshold voltage andlarge drain current, that is, with excellent characteristics necessaryfor switching elements.

Note that either of a negative resist material and a positive resistmaterial can be used as a resist material used in photolithography.

In the present embodiment, the description has been made on a method inwhich the resist layer 12 is formed on the plated film 11 that has beenprovided on a substrate, and then unnecessary portions of the platedfilm 11 are removed by etching, as a method of forming the source anddrain electrodes 3 and 4, the pixel electrodes 41, the connectionterminals 8 and the second lines 92. However, instead of this, thefollowing method may be used to form each element 3, 4, 41, 8 and 92.

Specifically, the resist layer 12 having openings that correspond to theshapes of the elements 3, 4, 41, 8 and 92 is formed on the substrate 2,and then the substrate 2 having the resist layer 12 thereon is immersedin the plating solution 13. Thus a plated film corresponding to theshapes of the elements 3, 4, 41, 8 and 92 is formed. Thereafter theresist layer 12 is removed, thereby resulting in the formation ofelements 3, 4, 41, 8 and 92.

(A2) Step of Removing Organic Substances

Subsequently, the substrate 2 having the source and drain electrodes 3and 4, the pixel electrodes 41, the connection terminals 8, and thesecond lines 92 formed thereon is cleaned with one or an adequatecombination of water (deionized water or the like), an organic solvent,etc.

Then, organic substances existing on the surface of the substrate 2 onwhich the organic semiconductor layer 5 is to be formed, are removed.This removes the barrier to carriers in the interface between theorganic semiconductor layer 5 to be formed in the later process and thesource and drain electrodes 3 and 4, enabling the improvement of thecharacteristics of the thin film transistor 1.

As a method of removing organic substances (removing method), forexample, plasma treatment, treatment with ozone water, etching with anacid or alkali, mechanical removal of a surface layer, and ultravioletrays (UV) (in particular, deep UV) radiation can be used. One or acombination of two or more of these methods can be used. Of thesemethods, plasma treatment is preferable as a method of removing organicsubstances. Plasma treatment enables organic substances to be removedsurely in a short time.

Plasma treatment may be implemented by carrying the substrate 2 in achamber equipped with a decompression means and a plasma generationmeans, and then generating plasma in the chamber in a decompressedcondition. Alternately, the plasma treatment may be implemented by usinga head that has plasma spray nozzles and splaying the substrate surfacewith plasma.

In the latter method, plasma treatment can be carried out in theatmospheric pressure (atmospheric pressure plasma treatment). Therefore,the use of a chamber and decompression means is unnecessary, and thusproviding advantages since manufacturing costs and time can be reduced.

In the case of atmospheric pressure plasma treatment, the conditionsthereof are as follows for example: the gas flow rate is about 10-300sccm, and the RF power is about 0.005-0.2 W/cm2.

A gas used for generating plasma is preferably, but not limited to, agas composed mainly of at least one of oxygen, nitrogen, argon, helium,and fluorocarbon. The mixing of argon or helium into the main componentenables plasma to be generated in a comparatively low vacuum atmosphereor under the atmospheric pressure, and thus the apparatus can besimplified. Note that the present step (A2) can be omitted according toneed.

(A3) Step of Forming Organic Semiconductor Layer

Then, as shown in FIG. 4C; the organic semiconductor layer 5 is formedon the substrate 2 on which the source and drain electrodes 3 and 4, thepixel electrodes 41, the connection terminals 8 and the second lines 92have been formed, so as to cover the source and drain electrodes 3 and4.

At this time, the channel regions 51 are formed on regions between thesource electrode fingers 3 a and the drain electrode fingers 4 a.

For example, the organic semiconductor layer 5 can be formed by applying(supplying) a solution including an organic polymer material or aprecursor thereof on the substrate 2 by using a coating method so as tocover the source and drain electrodes 3 and 4, and then carrying out apost-process (e.g., heating, irradiation with infrared rays, orprovision of ultrasonic waves) for the coating film according to need.

Examples of a coating method include, for example, a spin-coatingmethod, a casting method, a micro gravure coating method, a gravurecoating method, a bar coating method, a roller coating method, a wirebar coating method, a dip coating method, a spray coating method, ascreen printing method, a flexographic printing method, an offsetprinting method, an ink jet method, and a micro-contact printing method.One or a combination of two or more of these methods can be used.

Of these methods, an ink jet method is preferably used for forming theorganic semiconductor layer 5. Alternatively other methods may be used.An ink jet method allows the organic semiconductor layer 5 to beselectively formed so as to cover only the source and drain electrodes 3and 4, without supplying a resist layer or the like on the pixelelectrodes 41, the connection terminals 8 and the second lines 92. Thisreduces the amount of a consumed organic semiconductor material, andthereby reducing manufacturing costs.

Also, the region on which the organic semiconductor layer 5 is formed isnot limited to that shown in the drawings. The organic semiconductorlayer 5 may be formed only in the regions (channel region 51) betweenthe source electrode fingers 3 a and the drain electrode fingers 4 a.According to this structure, in the case of arranging a plurality ofthin film transistors 1 (elements) on a single substrate, leak currentsand cross-talk between elements can be suppressed by forming the organicsemiconductor layer 5 for each element independently of each other. Theuse of an ink jet method is also particularly suitable for forming theorganic semiconductor layer 5 only in the channel regions 51.Furthermore, the required resolution is 5-100 μm, and therefore matchesthe resolution by an ink jet method. The use of an ink jet method informing the organic semiconductor layer 5 for each element independentlyof each other eliminates the need to use chemicals such as photoresist,a developer or a remover, or plasma treatment such as oxygen plasma orCF4 plasma. Therefore, there are no concerns that the characteristics ofthe organic semiconductor material changes (for example, since thematerial is doped) or deteriorates.

In this case, for example, inorganic solvents, various organic solvents,mixed solvents including these solvents, or the like can be used as asolvent for dissolving the organic semiconductor material. Examples ofthe inorganic solvents include nitric acid, sulfuric acid, ammonia,hydrogen peroxide, water, carbon disulfide, carbon tetrachloride andethylene carbonate. Also, examples of the various organic solventsinclude: ketones such as methyl ethyl ketone (MEK), acetone, diethylketone, methyl isobutyl ketone (MIBK), methyl isopropyl ketone (MIPK)and cyclohexanon; alcohol solvents such as methanol, ethanol,isopropanol, ethylene glycol, diethylene glycol (DEG) and glycerine;ether solvents such as diethyl ether, diisopropyl ether,1,2-dimetoxyethane (DME), 1,4-dioxane, tetrahydrofuran (THF),tetrahydropyran (THP), anisole, diethylene glycol dimethyl ether(diglyme) and diethylene glycol ethyl ether (carbitol); cellosolvesolvents such as methyl cellosolve, ethyl cellosolve and phenylcellosolve; aliphatic hydrocarbon solvents such as hexane, pentane,heptane and cyclohexane; aromatic hydrocarbon solvents such as toluene,xylene and benzene; heteroaromatic solvents such as pyridine, pyrazine,furan, pyrrole, thiophene and methylpyrrolidone; amide solvents such asN, N-dimethylformamide (DMF) and N, N-dimethylacetamide (DMA);halogenated compound solvents such as dichloromethane, chloroform and 1,2-dichloroethane;

ester solvents such as ethyl acetate, methyl acetate and ethyl formate;sulfur compound solvents such as dimethyl sulfoxide (DMSO) andsulfolane; nitrile solvents such as acetonitrile, propionitrile andacrylonitrile; and organic acid solvents such as formic acid, aceticacid, trichloroacetic acid and trifluoroacetic acid.

An organic semiconductor material includes a conjugated system such asan aromatic hydrocarbon group or a heterocyclic group, and therefore isprone to be dissolved with an aromatic hydrocarbon solvent in general.Toluene, xylene, trimethylbenzene, tetramethylbenzene,cyclohexylbenzene, and and the like are particularly suitable solvents.

(A4) Step of Forming Gate Insulating Layer

Next, as shown in FIG. 4H, the gate insulating layer 6 is formed so asto cover the organic semiconductor layer 5 and the second lines 92, andalso the pixel electrodes 41 that are not shown in the drawing.

For example, the gate insulating layer 6 can be formed by applying(supplying) a solution including an insulating material or a precursorthereof on the organic semiconductor layer 5 by using a coating method,and then carrying out a post-process (e.g., heating, irradiation withinfrared rays, or provision of ultrasonic waves) for the coating filmaccording to need.

The same method as above can be used as the coating method. If theorganic semiconductor layer 5 is composed of a soluble organicsemiconductor material, a solvent that does not swell nor dissolve theorganic semiconductor layer 5 is preferably selected as the solvent forthe insulating material. As described above, since an organicsemiconductor material is easily dissolved with an aromatic hydrocarbonsolvent, it is more preferable to avoid the use of such solvents whencoating insulating materials. Namely, water solvents, alcohol solvents,ketone solvents, ether solvents, ester solvents, aliphatic hydrocarbonsolvents, or fluorinated solvents are preferably used.

Although the gate insulating layer 6 has a structure covering theorganic semiconductor layer 5, the pixel electrodes 41 and the secondlines 92 in the present embodiment, the gate insulating layer 6 is notlimited to such a structure but may be formed only on the organicsemiconductor layer 5. A spin coating method is suitable for the formerstructure, while an ink jet method is suitable for the latter. If thegate insulating layer 6 is formed by a spin coating method, theconnection terminals 8 are also covered. However, this covering may notbe desirable in a step of forming lines to be described next. Thus, itis useful to remove the gate insulating layer 6 on the connectionterminals 8, particularly on the first terminals 81 connected to thefirst lines 91. The following method can be used for this.

The connecting terminals 8 are previously covered (masking) with a resinadhesive tape before spin coating. Next, the adhesive tape is removedafter spin coating, whereby the connecting terminals 8 have no gateinsulating layer 6 thereon so as to be exposed.

Alternately, the gate insulating layer 6 can partially be removed bydropping a solution via an ink jet method on the gate insulating layer 6that has been formed over the entire surface by spin coating. In thiscase, the solution is dropped such that holes are opened in the parts ofthe gate insulating layer 6 that correspond to the connection terminals8. The dropped solution dries after dissolving the gate insulating layer6. At this time, the insulating material that has been once dissolved inthe solution re-precipitates in the periphery of the droplet of thesolution, and thus a hole is opened around the center of the droplet. Ifa hole does not penetrate to the connection terminal 8 with droplets ofone discharge, the repetition of dropping and drying of droplets for thesame place allows the hole to reach the connection terminal 8.

Also, instead of dropping a solution by an ink jet method above theconnection terminal 8 after alignment, it is also effective to dispersea solvent in a spraying manner for the removal of the gate insulatinglayer 6. This method uses a simpler device than the ink jet method,while providing high productivity. In order to limit areas sprayed withdroplets, a slit aperture may be inserted between spray nozzles and thedevice (circuit board 10).

It is also effective to use a needle tool for opening holes in the gateinsulating layer 6. If the gate insulating layer 6 is made of a polymer,the substrate 2 is a hard material such as glass, and the connectionterminals 8 are metal, it is particularly easy to make a hole. Morespecifically, since the gate insulating layer 6 is less hard than thesubstrate 2 and the connection terminals 8 in this case, perforating orscratching the gate insulating layer 6 with a metal probe pin permitsthe partial removal of the gate insulating layer 6. Since the substrateis made of a hard material, it is sufficient to control the pressure ofthe pin such that the pin itself is not damaged.

In contrast, if the substrate 2 is made of plastic, it is necessary tocontrol the pressure so as not to perforate the connection terminal 8with the pin. The control of the pin would be easy if the connectionterminal 8 is substantially thick (200 nm or more). Therefore, it iseffective to increase the thickness of metal layers only for theconnection terminals 8. The most suitable method for this purpose iselectroless or electrolytic plating. Plating with exposing only theconnection terminals 8 or immersing only the connection terminals 8 in aplating solution enables the thickness of metal layers to be at leastpartially increased.

Alternately, the gate insulating layer 6 that has been formed over theentire surface by spin coating may be exposed to plasma to be removed.If the gate insulating layer 6 is made of such a polymer as has beendescribed in the present embodiment, the gate insulating layer 6 canpartially be removed by placing the device (circuit board 10) in plasmain the presence of an oxygen gas or mixed gas of oxygen and CF4. In thiscase, however, since only the gate insulating layer 6 on the connectionterminals 8 needs to be removed partially, other parts should be masked.The gate insulating layer 6 on the connection terminals 8 can be removedby covering the organic transistor part with a metal plate mask and thenexposing the device to plasma. The plasma has no influence on otherparts, which are covered with the metal mask.

Alternately, if the connection terminals 8 are located at the edges ofthe substrate 2, immersing only the edges in a solvent to remove thegate insulating layer 6 is also available.

(A5) Step of Forming Lines (Gate Electrodes)

Next, as shown in FIG. 4I, the first line 91 (gate electrode 7) isformed on the gate insulating layer 6.

The first line 91 can be formed through the following method, forexample.

First a liquid material including electrical conductive particles issupplied in an almost straight line by using a coating method in orderto form the gate electrode 7 for the thin film transistors 1 arranged inone line, thereby forming a coating film.

The conductive particles are composed of the above-described conductivematerial (metal material or metal oxide material).

The same method as described above can be used as a coating method. Theuse of an ink jet method is particularly preferable. The ink jet methodenables the liquid material to be supplied accurately corresponding tothe first lines 91. Thus the first lines 91 can be formed with highdimensional accuracy.

The later post process (e.g., heating, irradiation with infrared rays,or provision of ultrasonic waves) for the coating film (liquid material)allows the formation of the first wires 91.

The case of using an ink jet method to form the first lines 91 will bedescribed below as an illustration.

The ink jet method allows patterning by discharging droplets of a liquidmaterial including conductive particles (it is referred to as inkhereinafter) from nozzles of a droplet discharge head.

The viscosity (at room temperature) of the ink is preferably, but notlimited to, about 2-20 cps in general, and is more preferably about 4-8cps. If the viscosity of the ink is within this range, droplets can morestably be discharged from nozzles. If the viscosity of the ink issmaller than the above range, the vibration of a piezo element, which isdisplaced when discharging, has a reduced tendency to be attenuated, andtherefore discharging is prone to be unstable. Meanwhile, if theviscosity of the ink is larger than the above range, the flow channelresistance of the ink is large. Therefore the supply of the ink becomesinsufficient in high-speed printing, causing unstable discharging.

Also, the volume (in average) of one ink droplet is preferably, but notlimited to, about 0.1-40 pL in general, and is more preferably about1-30 pL. Setting the volume (in average) of a single droplet within thisrange allows the formation of more accurate shapes. If the volume of anink droplet is too small, ink droplets needs to be discharged at highspeed. An ink jet head that satisfies this high-speed discharging isexpensive. In addition, obtaining required film thickness is alsodifficult and repeated printing is necessary, lowering the productivity.Meanwhile, if an ink droplet is too large in size, the resolution ofprinting is lowered, as is obvious, making the features of an ink jetmethod ineffective.

Such materials as will be described in <A> and <B> below, for example,can be used as the ink.

<A> In the case of making up the first lines 91 (gate electrode 7) of ametal material or metal oxide material, a dispersion liquid includingconductive particles (metal particles or metal oxide particles) can beused as an ink (liquid material).

In this case, the content of the conductive particles in the ink ispreferably, but not limited to, about 1-40 wt %, and is more preferablyabout 10-30 wt %.

Also, the average particle diameter of the used conductive particles ispreferably, but not limited to, about 1-100 nm, and is more preferablyabout 2-30 nm.

Furthermore, particles coated with a coagulation inhibition agent(dispersant) for inhibiting coagulation at a room temperature arepreferably used for the conductive particles. As the coagulationinhibition agent, for example, compounds having a group including anitrogen atom such as alkylamine, compounds having a group including anoxygen atom such as alkanediol, and compounds having a group including asulfur atom such as alkylthiol and alkanethiol can be used. Propyleneglycol, trimethylene glycol, ethylene glycol or butanediol is preferableas an alkanediol.

In this case, a remover capable of removing the coagulation inhibitionagent by a predetermined process (e.g., heating or the like) is addedinto the ink. For example, examples of the remover include variouscarboxylic acids such as linear or branched saturated carboxylic acidsof one to ten carbon atoms, unsaturated carboxylic acids and dibasicacids. As the saturated carboxylic acids, a formic acid, acetic acid,propionic acid, butanoic acid, hexanoic acid and octylic acid may beused. As the unsaturated carbonic acids, an acrylic acid, methacrylicacid, crotonic acid, cinnamic acid, benzoic acid and sorbic acid may beused. As the dibasic acids, an oxalic acid, malonic acid, sebacic acid,maleic acid, fumaric acid and itaconic acid may be used. Also, examplesof the remover include organic acids such as various phosphoric acidsand various sulfonic acids that are obtained by substituting phosphategroups or sulfonyl groups for carboxyl groups of the above carboxylicacids, and organic acid esters derived from the organic acids. Inaddition, examples of the remover also include: aromatic acid anhydridessuch as phthalic anhydride, trimellitic anhydride, pyromelliticdianhydride, benzophenone tetracarboxylic dianhydride, ethylene glycolbis (anhydro trimellitate) and glycerol tris (anhydro trimellitate);cyclic fatty acid anhydrides such as maleic anhydride, succinicanhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalicanhydride, methyl nadic anhydride, alkenyl succinic anhydride,hexahydrophthalic anhydride, methylhexahydrophthalic anhydride andmethylcyclohexene tetracarboxylic dianhydride; and fatty acid anhydridessuch as poly(adipic anhydride), poly(azelaic anhydride) and poly(sebacicanhydride).

As a dispersion medium, for example, terpineol, mineral spirit, xylene,toluene, ethyl benzene, mesitylene, hexane, heptane, octane, decane,dodecane, cyclohexane, cyclooctane, ethanol, isopropanol (IPA), water,or a mixed liquid including these substances can be used. Of thesesubstances, a dispersion medium including water is preferably used inparticular.

Furthermore, precursors of various thermoset resins such as phenolresin, epoxy resin, unsaturated polyester resin, vinylester resin,diallyl phthalate resin, oligoester acrylate resin, xylene resin,bismaleimide triazine resin, furan resin, urea resin, polyurethaneresin, melamine resin, or silicone resin may be added into (mixed with)the ink.

Note that the viscosity of the ink can be adjusted by, for example,adequately setting the content of conductive particles, the kind orcomposition of a dispersion medium, and the presence or absence, or thekind of additives.

<B> In the case of making up the first lines 91 (gate electrode 7) of ametal material, a dispersion liquid including a reducing agent and metaloxide particles composed of a metal oxide material that is reduced so asto become a metal material can be used as the ink.

In this case, the content of the metal oxide particles in the ink ispreferably, but not limited to, about 1-40 wt %, and is more preferablyabout 10-30 wt %.

Also, the average particle diameter of the used metal oxide particles ispreferably, but not limited to, 100 nm or less, and is more preferably30 nm or less.

As the reducing agent, for example, an ascorbic acid, hydrogen sulfide,oxalic acid, and carbon monoxide can be used.

As the dispersion medium, for example, a low viscosity oil such as butylcellosolve or polyethylene glycol, an alcohol such as 2-propanol, or amixed liquid including these substances can be used.

Note that the viscosity of the ink can be adjusted by, for example,adequately setting the content of the metal oxide particles, the kind orcomposition of the dispersion medium, etc.

Then, post treatment (e.g., heating, irradiation with infrared rays, orprovision of ultrasonic waves) is implemented for the coating film(liquid material) that has been supplied on the gate insulating layer 6while corresponding to the shape of the first lines 91, thereby formingthe first lines 91.

Although a method for the post treatment is not particularly limited aslong as the method can remove substances (e.g., a dispersion medium,coagulation inhibition agent, and remover) that are included in theliquid material and should be removed, a method employing heat treatmentis preferably used. Heat treatment enables such substances to be removedeasily and surely. As a result, a porous film (first line 91) havinghigher porosity is obtained.

Heating temperature in the heat treatment is preferably 200° C. or less,and is more preferably 150° C. or less. Also, it is further preferably120° C. or less. This temperature range prevents the lowering of theporosity due to the excessive densification of the porous film.

In addition, setting of such heating temperature allows a part of thesubstances that should be removed to turn to the above-described carbonatoms or carbon compounds so as to remain in pores of the porous film(first line 91). Thus, such a method can reduce processes and thereforeis advantageous compared with a process in which such carbon atoms orcarbon compounds are provided in pores after a porous film is formed.

Through the above steps, the circuit board 10 including a plurality ofthin film transistors 1 that is shown in FIG. 2 is obtained.

In such a manufacturing method, the electroless plating is used as amethod of forming the source and drain electrodes 3 and 4 while theorganic semiconductor layer 5, the gate insulating layer 6, and the gateelectrode 7 are formed by a coating method. Therefore, the thin filmtransistor 1 can be manufactured with simple processes at lower costswithout requiring any large-scale apparatus such as a vacuum apparatus.

Also, by using electroless plating as a method of forming the source anddrain electrodes 3 and 4, these electrodes can be formed with highdimensional accuracy, resulting in the fabrication of the thin filmtransistor 1 having superior characteristics (switchingcharacteristics).

Furthermore, by using a plating solution substantially free of alkalimetal ions as the plating solution 13 used for the electroless plating,the mixing of alkali metal ions into the source and drain electrodes 3and 4 can be prevented, and thus avoiding the diffusion of alkali metalions into the organic semiconductor layer 5. This avoids thedeterioration of the characteristics of the organic semiconductor layer5, with the result that the thin film transistor 1 having superiorcharacteristics required for a switching element can be manufactured.

Moreover, the implementation of the step of removing organic substancesdescribed in step (A2) further enhances the characteristics of the thinfilm transistor 1.

Furthermore, by using atmospheric pressure plasma as the method ofremoving organic substances in step (A2) while using wet-etching as themethod of removing the plated film 11 in step (A1), all manufacturingsteps for the thin film transistor 1 can be carried out under theatmospheric pressure, and therefore the manufacturing costs and time canbe reduced.

Display

A display incorporating the above-described circuit board 10 will now bedescribed with taking an electrophoretic display as an example.

FIG. 5 is a longitudinal sectional view showing an embodiment ofapplying the circuit board 10 of the present invention to anelectrophoretic display.

An electrophoretic display 20 shown in FIG. 5 is composed of the circuitboard 10 and an electrophoretic display part 25 provided on the circuitboard 10.

As shown in FIG. 5, the electrophoretic display part 25 includes afacing substrate 251, a counter electrode 252, microcapsules 40 and abinder 45.

The counter electrode 252 is deposited on the facing substrate 251. Themicro capsules 40 (display media) are fixed onto the counter electrode252 with the binder 45.

The pixel electrodes 41 included in the circuit board 10 are separatedin a matrix, that is, in such a manner of being arranged regularly alongthe vertical and horizontal directions. Each pixel electrode 41 isconnected to the drain electrode 4 of the thin film transistor 1 and iscovered by the gate insulating layer 6.

The electrophoretic display part 25 is joined to the circuit board 10with the intermediary of a protective film 30 that is provided closer tothe circuit board 10 than the microcapsules 40 (display media) so as tocover the thin film transistors 1 and the pixel electrodes 41.

The protective film 30 has functions to protect the thin film transistor1 mechanically, and to prevent the diffusion of organophilic liquidstoward the circuit board 10.

Each of the capsules 40 contains an electrophoretic dispersion liquid400 including plural kinds of electrophoretic particles having differentcharacteristics from each other. In the present embodiment, theelectrophoretic dispersion liquid 400 includes two kinds ofelectrophoretic particles 401 and 402 having opposite charge polaritiesand different colors (hues).

Also, terminals of an IC for driving are connected to the connectionterminals 8 (terminals 81 and 82) included in the circuit board 10,whereby ON/OFF switching of the thin film transistor 1 (switchingelement) incorporated in the circuit board 10 is possible.

Specifically, in such an electrophoretic display 20, the supply ofselection signals (selection voltages) to one or more of the lines 91allows the thin film transistors 1 connected to the lines 91 to whichthe selection signals (selection voltages) have been supplied, to beswitched ON.

Thus, the lines 92 and the pixel electrodes 41 connected to such thinfilm transistors 1 are electrically connected to each othersubstantially. If desired data (voltage) is supplied to the lines 92 inthis state, the data (voltage) is supplied to the pixel electrodes 41.

At this time, an electric field is generated between the pixelelectrodes 41 and the counter electrode 252. The electrophoreticparticles 401 and 402 are electrophoresed toward either of theelectrodes in accordance with the direction and strength of the electricfield, the characteristics of the electrophoretic particles 401 and 402,and and the like.

Meanwhile, in this state, when the supply of the selection signals(selection voltages) to the lines 91 is stopped, the thin filmtransistors 1 are switched OFF and therefore the lines 92 and the pixelelectrodes 41 connected to the thin film transistors 1 are electricallyisolated from each other.

Therefore, adequate combinations of the supply and stop of selectionsignals to the lines 91, and that of data to the lines 92 permit thedisplaying of desired images (information) on the display surface side(facing substrate) of the electrophoretic display 20.

In particular, in the electrophoretic display 20 of the presentembodiment, the electrophoretic particles 401 and 402 are differentlycolored, and thus enabling images of multiple grayscale to be displayed.

Furthermore, since the electrophoretic display 20 of the presentembodiment is equipped with the circuit board 10, the thin filmtransistors 1 connected to the specific lines 91 can selectively beswitched ON/OFF. Therefore, the problem of cross-talk is hardly causedand the speed of the circuit operation can be increased, and thus highquality images (information) can be obtained.

In addition, the electrophoretic display 20 of the present embodiment isoperated with lower drive voltages, enabling lower power consumption.

Such an electrophoretic display 20 can be manufactured as follows, forexample.

First the circuit board 10 and the electrophoretic display part 25 arepreviously manufactured.

Then, the protective film 30 is formed on at least one of the surface ofthe circuit board 10 on the gate insulating film 6 side and the surfaceof the electrophoretic display part 25 on the microcapsules 40 side.

Subsequently, for example, the circuit board 10 and the electrophoreticdisplay part 25 are pressed so as to become close to each other whileheat is applied thereto, in a state in which the gate insulating film 6of the circuit board 10 is contacted to the microcapsules 40 of theelectrophoretic display part 25 with the intermediary of the protectivefilm 30. Thus, the protective film 30 functions as a binder andtherefore the circuit board 10 is joined to the electrophoretic displaypart 25, manufacturing the electrophoretic display 20.

For the electrophoretic dispersion liquid 400 contained in each capsule40 (display medium), the above-described aromatic hydrocarbons are usedas a dispersion medium. These substances are liquids having highorganophilicity (lipid solubility).

In the case of omitting the formation of the protective film 30,pressing the circuit board 10 and the electrophoretic display 25 inmanufacturing the electrophoretic display 20 causes pressure and shearstress to be directly placed on the thin film transistors 1, themicrocapsules 40, etc. If the microcapsules 40 burst (are damaged) inthe pressing, the electrophoretic dispersion liquid 400 in themicrocapsules 40 flows out.

Since the dispersion medium included in the electrophoretic dispersionliquid 400 has high organophilicity (lipid solubility), if theelectrophoretic dispersion liquid 400 flows outside the microcapsules40, the electrophoretic dispersion liquid 400 infiltrates (diffuses)into the gate insulating layer 6 and the organic semiconductor layer 5so as to dissolve or swell the component materials of these elements. Ifthe component material (organic semiconductor material) of the organicsemiconductor layer 5 is dissolved or swelled in particular, thecharacteristics of the thin film transistor 1 deteriorate.

In contrast, the electrophoretic display 20 has the protective film 30.Therefore, even if the microcapsules 40 burst and the electrophoreticdispersion liquid 400 flows out, the presence of the protective film 30prevents the organic semiconductor layer 5 from dissolving or swelling.Thus, the deterioration of the characteristics of the thin filmtransistor 1 can be prevented adequately.

A material composed mainly of a hydrophilic material, particularly ahydrophilic polymer material is preferable for the component material ofthe protective film 30. By using such a polymer material, the formationof the protective film 30 is facilitated while it can adequately beprevented or suppressed that shear stress is placed on the thin filmtransistors 1, the microcapsules 40, etc. Thus damages to the thin filmtransistors 1, the microcapsules 40 and so forth can more surely beavoided.

As a polymer material having hydrophilicity, for example, a materialcomposed mainly of a polymer including vinyl alcohol such as polyvinylalcohol, ethylene-vinyl alcohol copolymer, vinyl chloride-vinyl alcoholcopolymer or vinyl acetate-vinyl alcohol copolymer is preferable. Thesesubstances have particularly high hydrophilicity and therefore can moresurely prevent the entrance of the organophilic liquid into the circuitboard 10 through the protective film 30.

If a plastic material is used for the substrate 2, the transmittance ofthe substrate 2 for the organophilic liquid can be set higher than thatof the protective film 30 for the organophilic liquid. In this case, therisk of the organophilic liquid vapor accumulating in the organicsemiconductor layer 5 is reduced. Specifically, although a small amountof the organophilic liquid vapor that has permeated the protective film30 passes through the porous gate electrode so as to reach the organicsemiconductor layer 5, the vapor is immediately discharged from thesubstrate 2 side.

The electrophoretic display 20 of the present embodiment has a structurein which a plurality of microcapsules 40 enclosing the electrophoreticdispersion liquid 400 is interposed between the pixel electrodes 41 andthe counter electrode 252. However, the electrophoretic display 20 mayhave a structure in which a plurality of spaces (cells) is delimitedwith partitions and the electrophoretic dispersion liquid 400 isenclosed in each space. The electrophoretic display 20 having such astructure also adequately exerts the above-described operations andadvantageous effects.

Note that the display of the present invention is not limited to theapplication to such an electrophoretic display 20, but can also beapplied to liquid crystal displays, organic or inorganic EL displays andthe like.

Electronic Equipment

Such an electrophoretic display 20 can be incorporated in variouselectronic equipment. Electronic equipment of the present inventionequipped with the electrophoretic display 20 will be described below.

Electronic Paper

First an embodiment of applying the electronic equipment of the presentinvention to an electronic paper will now be explained.

FIG. 6 is a perspective view showing an embodiment of applyingelectronic equipment of the present invention to an electronic paper.

An electronic paper 600 shown in this drawing is equipped with a mainbody 601 composed of a rewritable sheet having the same texture andflexibility as a paper, and a display unit 602.

In such an electronic paper 600, the display unit 602 is composed of theabove-described electrophoretic display 20.

Display

Next, an embodiment of applying the electronic equipment of the presentinvention to a display will be described.

FIGS. 7A and 7B are diagrams showing an embodiment of applying theelectronic equipment of the present invention to a display, where FIG.7A is a cross-sectional view and FIG. 7B is a plan view.

A display 800 shown in the drawing is equipped with a main body 801 andthe electronic paper 600 detachably provided in the main body 801. Theelectronic paper 600 has the above-described structure, that is, thesame structure as that shown in FIG. 6.

The main body 801 is provided with an inlet 805 at a side thereof (rightside in the figure) through which the electronic paper 600 can beinserted, and two pairs of feed rollers 802 a and 802 b inside thereof.When the electronic paper 600 is inserted into the main body 801 throughthe inlet 805, the electronic paper 600 is placed in the main body 801while being interposed between the feed rollers 802 a and 802 b.

Also, on the display surface side (near side of FIG. 7B) of the mainbody 801, there is formed a rectangular hollow part 803 in which atransparent glass plate 804 is engaged. With such a structure, theelectronic paper 600 placed in the main body 801 can be viewed from theoutside of the main body 801. That is, the display 800 makes theelectronic paper 600 placed in the main body 801 be viewed through thetransparent glass substrate 804, thereby offering a display surface.

Furthermore, a terminal part 806 is provided on a tip of the electronicpaper 600 at the end of the inserting direction (left side in thedrawing), and a socket 807 to which the terminal part 806 is connectedwhile the electronic paper 600 is placed in the main body 801, isprovided inside the main body 801. A controller 808 and an operatingunit 809 are electrically connected to the socket 807.

In such a display 800, the electronic paper 600 is detachably providedin the main body 801 and therefore can also be used with being detachedfrom the main body 801 and being carried by a user.

Also, in such a display 800, the electronic paper 600 is composed of theabove-described electrophoretic display 20.

Note that the application of the electronic equipment of the presentinvention is not limited to those described above. For example, examplesof the application include a television, a video tape recorder of aview-finder type or a monitor viewing type, a car navigation system, apager, a personal digital assistance, an electronic calculator, anelectronic newspaper, a word processor, a personal computer, aworkstation, a picture phone, a POS terminal, and a device equipped witha touch panel. The electrophoretic display 20 can be applied to adisplay of the above various electronic equipment.

Although the above descriptions have been made on a thin filmtransistor, a circuit board, a display and electronic equipment of thepresent invention, it should be understood that the present invention isnot limited to the descriptions.

For example, although a thin film transistor of a top gate structure hasbeen described in the above embodiment, the present invention can alsobe applied to thin film transistors of a bottom gate structure.

In addition, the structure of each part of the thin film transistor,circuit board, display and electronic equipment of the present inventioncan be replaced with any structure capable of exhibiting the samefunctions. Alternately, any structure can be added thereto.

WORKING EXAMPLES

Specific working examples of the present invention will now bedescribed.

In the specific working examples, a thin film transistor wasmanufactured by using the above-described method of manufacturing acircuit board.

Therefore, methods of manufacturing thin film transistors and theevaluation of the manufactured thin film transistors will be describedbelow.

1. Manufacturing of thin film transistor

In the following examples, deionized water is used as water unlessotherwise noted.

First Working Example

First a glass substrate with an average thickness of 1 mm was preparedand cleaned with water (cleaning fluid).

Next, the glass substrate was immersed in an aqueous solution (25° C.)of distearyl dimethyl ammonium chloride (cationic surfactant) for 60seconds. Thus, distearyl dimethyl ammonium chloride was adsorbed to thesurface of the glass substrate. Subsequently, the glass substrate wascleaned with water.

Then, the glass substrate was immersed in a Sn—Pd colloidal suspension(25° C.) for 60 seconds. Thus, Sn—Pd was adsorbed to the surface of theglass substrate. Subsequently, the glass substrate was cleaned withwater.

Then, the glass substrate was immersed in an aqueous solution (25° C.)including HBF4 and glucose for 60 seconds. Thus, Sn was removed from thesurface of the glass substrate to expose Pd on the surface of the glasssubstrate. Subsequently, the glass substrate was cleaned with water.

Then, the glass substrate was immersed in a Ni plating solution (80° C.,pH 8.5) for 60 seconds. Thus, a Ni plated film with an average thicknessof 100 nm was formed on the surface of the glass substrate.

The Ni plating solution was prepared by dissolving 10 g of nickelsulfate, 100 g of hydrazine (reducing agent) and 5 g of ammonium sulfide(pH adjuster) in 1 L of water.

Next, a resist layer patterned correspondingly to the shapes of sourceand drain electrodes was formed on the Ni plated film byphotolithography.

“OPR800” produced by Tokyo Ohka Kogyo Co., Ltd was used as the resistmaterial.

Then, the glass substrate was immersed in an aqueous solution (25° C.)of ferric chloride. Thus, the plated film that was not covered by theresist layer was removed to form a source electrode and drain electrode.

The distance (channel length L) between source electrode fingers anddrain electrode fingers was set to 10 μm, and the channel width W wasset to 1 mm.

Then, after removing the resist layer by using a resist remover, theglass substrate having the source and drain electrodes formed thereonwas cleaned with water and methanol sequentially.

Then, oxygen plasma treatment (atmospheric pressure oxygen plasmatreatment) was carried out for the glass substrate having the source anddrain electrodes formed thereon under the atmospheric pressure.

The conditions of the atmospheric pressure plasma treatment were asfollows: RF power was 0.05 W/cm2 and gas flow rate was 80 sccm.

Next, a trimethylbenzene solution of 1.5% (wt/vol) polyphenylamine wasapplied on the glass substrate by an ink jet method (volume of onedroplet was 20 pL), and then was dried at 60° C. for 10 minutes. Thus,an organic semiconductor layer with an average thickness of 50 nm wasformed.

Subsequently, a butyl acetate solution of 6% (wt/vol) polystyrene wasapplied on the organic semiconductor layer by spin coating (2400 rpm)and then was dried at 60° C. for 10 minutes. Thus, the gate insulatinglayer with an average thickness of 400 nm was formed.

Then, a water dispersion liquid of Ag particles (viscosity of 6 cps atroom temperature) was applied by an ink jet method (volume of onedroplet was 20 pL) on part on the gate insulating layer thatcorresponded to the region in which the source electrode fingers and thedrain electrode fingers interdigitated with each other, and thereafterwas baked at 120° C. for 60 minutes. Thus, a gate electrode with anaverage thickness of 810 nm (porosity of 61%) was formed.

The thin film transistor shown in FIGS. 1A and 1B was manufacturedthrough the above processes.

Second Through Fifth Working Examples

The thin film transistors shown in FIGS. 1A and 1B were manufactured inthe same way as the first working example except that baking conditionfor a water dispersion liquid of Ag particles supplied on the gateinsulating layer and the average thickness of the formed gate electrodewere varied as shown in Table 1.

Sixth Working Example

Source and drain electrodes were formed in the same way as the firstworking example except that a polyimide substrate with an averagethickness of 35 μm was used instead of a glass substrate. Thereafter, atoluene solution of 1% (wt/vol) fluorene-bithiophene copolymer wasapplied on the polyimide substrate by an ink jet method (volume of onedroplet was 20 pL) and then was dried at 60° C. for 10 minutes. Thus, anorganic semiconductor layer with an average thickness of 50 nm wasformed.

Subsequently, a gate insulating layer and a gate electrode were formedin the same way as the above first working example, manufacturing thethin film transistors shown in FIGS. 1A and 1B.

Seventh Through Tenth Working Examples

The thin film transistors shown in FIGS. 1A and 1B were manufactured inthe same way as the sixth working example except that baking conditionfor a water dispersion liquid of Ag particles supplied on the gateinsulating layer and the average thickness of the formed gate electrodewere varied as shown in Table 1.

Comparative Example 1

A source and drain electrodes, organic semiconductor layer and gateinsulating layer were manufactured in the same way as the first workingexample. Then, a resist layer having a pattern that corresponds to aregion in which a gate electrode is not formed was formed on the gateinsulating layer by photolithography.

“OPR800” produced by Tokyo Ohka Kogyo Co., Ltd. was used as the resistmaterial.

Subsequently, a Ag film (gate electrode) was formed by vacuum deposition(the pressure inside the chamber was 1×10-3 Torr, the heatingtemperature of the substrate was 100° C.) on part on the gate insulatinglayer that corresponded to a region in which the resist layer is notformed, that is, a region in which the source electrode fingers and thedrain electrode fingers interdigitate with each other.

Then, after removing the resist layer by using a resist remover, theglass substrate having the gate electrode formed on the gate insulatinglayer was cleaned with water and methanol sequentially. Thus, the gateelectrode with an average thickness of 800 nm was formed.

The thin film transistor shown in FIGS. 1A and 1B was manufacturedthrough the above processes.

Second Working Example

Source and drain electrodes were formed in the same way as the firstcomparative example except that a polyimide substrate with an averagethickness of 35 μm was used instead of a glass substrate. Thereafter, atoluene solution of 1% (wt/vol) fluorene-bithiophene copolymer wasapplied on the polyimide substrate by an ink jet method (volume of onedroplet was 20 μL) and then was dried at 60° C. for 10 minutes. Thus, anorganic semiconductor layer with an average thickness of 50 nm wasformed.

Subsequently, a gate insulating layer and a gate electrode ware formedin the same way as the above first comparative example, manufacturingthe thin film transistor shown in FIGS. 1A and 1B. TABLE 1 gate organicelectrode gate electrode semi- (Ag) average conductor baking thicknessporosity substrate layer condition [nm] [%] working glass polyphenyl-120° C. 810 61 example 1 substrate amine 60 min in the air working glasspolyphenyl- 130° C. 680 54 example 2 substrate amine 60 min in the airworking glass polyphenyl- 150° C. 610 47 example 3 substrate amine 60min in the air working glass polyphenyl- 170° C. 560 35 example 4substrate amine 60 min in the air working glass polyphenyl- 200° C. 49022 example 5 substrate amine 60 min in the air working polyimidefluorene- 120° C. 800 60 example 6 substrate bithiophene 30 mincopolymer in the air working polyimide fluorene- 130° C. 690 51 example7 substrate bithiophene 30 min copolymer in the air working polyimidefluorene- 150° C. 600 45 example 8 substrate bithiophene 30 mincopolymer in the air working polyimide fluorene- 170° C. 570 33 example9 substrate bithiophene 30 min copolymer in the air working polyimidefluorene- 200° C. 500 26 example 10 substrate bithiophene 30 mincopolymer in the air comparative glass polyphenyl- (vacuum 800 — example1 substrate amine depo- sition) comparative polyimide fluorene- (vacuum800 — example 2 substrate bithiophene depo- copolymer sition)

2. Evaluation

The thin film transistors manufactured in each working example andcomparative example were subjected to (1) a low temperature and lowhumidity environment (20° C., 30% RH) for one hour, and then weresubjected to (2) a high temperature and high humidity environment (80°C., 80% RH) for 30 minutes. Thereafter the thin film transistors wereagain subjected to (3) a low temperature and low humidity environment(20° C., 30% RH) for five hours.

With respect to the thin film transistors manufactured in each workingexample and comparative example that had been subjected to suchenvironments, the off-state current was measured after each of the abovethree stages.

The term off-state current refers to a current flowing between a sourceand drain electrodes when a voltage is not applied to a gate electrode.

Therefore, an off-state current closer to zero indicates that the thinfilm transistor has better characteristics.

These off-state currents are shown in Table 2. TABLE 2 off-state current[nA/μm] (supply voltage = 1.2 V) (1): (2): (3): 20° C., 30% RH 80° C.,80% RH 20° C., 30% RH for 1 hour for 30 minutes after for 5 hours (1)after (2) working 5 × 10⁻² 4 × 10² 7 × 10⁻² example 1 working 5 × 10⁻² 4× 10² 8 × 10⁻² example 2 working 6 × 10⁻² 2 × 10² 9 × 10⁻² example 3working 5 × 10⁻² 3 × 10² 9 × 10⁻² example 4 working 6 × 10⁻² 2 × 10³ 1 ×10⁻¹ example 5 working 7 × 10⁻² 6 × 10² 9 × 10⁻² example 6 working 7 ×10⁻² 5 × 10² 9 × 10⁻² example 7 working 6 × 10⁻² 5 × 10² 2 × 10⁻¹example 8 working 7 × 10⁻² 5 × 10² 2 × 10⁻¹ example 9 working 7 × 10⁻² 4× 10² 3 × 10⁻¹ example 10 comparative 5 × 10⁻² 2 × 10¹ 1 × 10² example 1comparative 7 × 10⁻² 4 × 10¹ 3 × 10² example 2

As shown in Table 2, all thin film transistors manufactured in theworking examples and comparative examples had a small off-state currentand excellent characteristics at the time when the thin film transistorshad been subjected to (1) a low temperature and low humidity environment(20° C., 30% RH) for one hour.

Subsequently, in the thin film transistors of the working examples, anoff-state current was increased and the characteristics deterioratedafter the thin film transistors had been subjected to (2) a hightemperature and high humidity environment (80° C., 80% RH) for 30minutes. However, after the thin film transistors had been againsubjected to (3) a low temperature and low humidity environment (20° C.,30% RH) for five hours, the off-state current almost reverted to thevalue that had been measured after (1). This result indicates thatoxygen and water that has been trapped in the thin film transistors aresmoothly discharged. This discharging may allow the characteristics ofthe thin film transistors to be recovered rapidly.

In contrast, in all thin film transistors manufactured in thecomparative examples, after the thin film transistors had been subjectedto (2) a high temperature and high humidity environment (80° C., 80% RH)for 30 minutes, the increase of an off-state current was smaller thanthat for the working examples. However, even after the thin filmtransistors had been again subjected to (3) a low temperature and lowhumidity environment (20° C., 30% RH) for five hours, the off-statecurrent was not decreased (recovered) but had a tendency to be increasedconversely. This indicates that oxygen and water accumulate in the thinfilm transistor.

REFERENCE NUMERALS

-   1: thin film transistor,-   2: substrate,-   3: source electrode, 3 a: electrode finger,-   4: drain electrode,-   4 a: electrode finger, 41: pixel electrode,-   5: organic semiconductor layer, 51: channel region,-   6: gate insulating layer,-   7: gate electrode,-   8: connection terminal, 81: first terminal, 82: second terminal,-   9: line, 91: first line, 92: second line,-   10: circuit board, 11: plated film, 12: resist layer, 12′: resist    material, 13: plating solution,-   20: electrophoretic display,-   25: electrophoretic display part, 251: facing substrate, 252:    counter electrode,-   30: protective film,-   40: microcapsule, 400: electrophoretic dispersion liquid, 401, 402:    electrophoretic particle,-   45: binder,-   600: electronic paper, 601: main body, 602: display unit,-   800: display, 801 main body, 802 a, 802 b: pair of feed roller, 803:    hollow part, 804: transparent glass plate, 805: inlet, 806: terminal    part, 807: socket, 808: controller, 809: operating unit

1. A transistor comprising: a source electrode and a drain electrode; anorganic semiconductor layer provided in contact with the sourceelectrode and the drain electrode; a gate insulating layer provided incontact with the organic semiconductor layer; and a gate electrodeinsulated from the source electrode and the drain electrode byintermediary of the gate insulating layer, wherein at least one of thegate electrode, the source electrode and the drain electrode is composedof a porous film including an electrically conductive material.
 2. Thetransistor according to claim 1, the transistor having a top gatestructure, and the gate electrode being composed of the porous film. 3.The transistor according to claim 1, porosity of the porous film beingfrom 20% to 85%.
 4. The transistor according to claim 1, the porous filmhaving a carbon atom or a carbon compound in a pore of the porous film.5. The transistor according to claim 1, the conductive materialcomprising a metal material and/or a metal oxide material as a maincomponent.
 6. The transistor according to claim 1, the organicsemiconductor layer being composed mainly of a polymer includingarylamine or a derivative of the polymer.
 7. The transistor according toclaim 1, the organic semiconductor layer being composed mainly of acopolymer including fluorene and bithiophene, or a derivative of thecopolymer.
 8. The transistor according to claim 1, both of the sourceelectrode and the drain electrode being formed in a comb teeth shape,the teeth of the source electrode and the drain electrode beinginterdigitated with each other.
 9. A circuit board having a transistoraccording to claim
 1. 10. A display having the circuit board accordingto claim
 9. 11. Electronic equipment having the display according toclaim 10.